1. Field
Embodiments of the disclosure relate to electronic devices, and more particularly, in one or more embodiments, to wireless transmitters.
2. Description of the Related Technology
Transmitters for wireless infrastructure (e.g., cellular basestations) have traditionally been implemented using super-heterodyne or complex intermediate frequency (IF) architectures. Implementing a wireless transmitter (TX) using a direct conversion architecture instead of a super-heterodyne architecture can reduce overall system cost and size through integration and the use of fewer components. However, several issues can arise with the use of direct conversion wireless transmitters.
Direct conversion transmitters (TX) include an in-phase (I) and quadrature phase (Q) baseband path, each driving a mixer also driven by a local oscillator (LO) signal having a frequency about equal to the desired radio frequency (RF) center frequency. The I-path mixer LO signal and Q-path mixer LO signal are 90 degrees out of phase (sine and cosine), and the mixer outputs are summed at RF. Any mismatch in the amplitude of the I or Q path (amplitude error), or any deviation of the phase difference of the two paths from 90 degrees (phase error) is referred to collectively as quadrature error or a quadrature imbalance. Quadrature errors can result in an undesired sideband (USB), in which a desired signal at some frequency offset from the carrier will have an undesired image at the negative of that offset frequency. This image is considered an undesired emission, and acceptable levels of undesired emissions are determined by various wireless standards.
In some instances, for multi-carrier (MC) basestation applications, a direct conversion TX is required to have very low undesired sideband levels (e.g., less than −75 dBm/Hz), which can be achieved through a calibration process called quadrature error correction (QEC).
Some QEC processes observe the transmit signal alone (blind algorithms) and assume zero correlation between the I and Q TX signals when the undesired sideband has been eliminated. However, digital pre-distortion (DPD), which is used in basestations to improve power amplifier (PA) efficiency, can create correlation between the I and Q TX signals. Thus, for blind QEC processes, the DPD-related correlation can be falsely detected as a quadrature error, and limit the minimum achievable undesired sideband.
To further complicate QEC, quadrature errors in the transmitter can vary with baseband frequency due to baseband filter mismatch, DAC clock skew, etc. In addition, placing the transmitter offline to complete the calibration process is undesirable as it can lead to dropped calls and other undesirable side effects.